Built-in FM dipole antenna

ABSTRACT

A component type of FM stereo broadcast receiver is provided with a built-in dipole antenna by making use of the cables that connect the left and right speakers to the receiver. Since these speakers are usually placed at least one half wavelength apart at the received carrier frequencies, the system lends itself to efficient dipole antenna operation. By using an r-f impedance raising circuit at the amplifier output, and a frequency selective coupling system between the speaker jacks and the antenna terminals, a high performance &#39;&#39;&#39;&#39;built-in&#39;&#39;&#39;&#39; antenna is achieved.

finite Sttes Paten 11 1 Quinlan et al.

[ Jan. 28, 1975 BUILT-IN FM DlPOLE ANTENNA [73] Assignee: Philco-Ford Corporation, Blue Bell.

22 Filed: Aug. 27, 1973 211 App]. No.: 391,991

OTHER PUBLICATIONS Sony Stereo Receiver, STR-6050, Owner's Manual, April l, 1970.

Primary Iiranzirwr-Robcrt L. Griffin Assistant Iiruminer-Jin F. Ng

Attorney. Agent, or FirmRobert D. Sanborn; Gail W. Woodward 52 us. or. 325/365, 343/702 ABSTRACT [51] hit. Cl. H04q 1/24 A omponent type of FM tereo broadcast receiver is l l Fleld of Search provided with a built-in dipole antenna by making use 6, 179/1 15 of the cables that connect the left and right speakers 1 GA to the receiver.v Since these speakers are usually placed at least one half wavelength apart at the re- [56] Refer n Cit ceived carrier frequencies, the system lends itself to UNITED STATES PATENTS efficient dipole antenna operation. By using an r-f imedance raisin circuit at the am lifier out ut, and a 2,581,983 1 1952 Thompson 325/365 P g P P 3,386,033 5/1968 Copeland et al 325/373 frequency selecflveh Couplmg System between the speaker jacks and t e antenna terminals, a high per- FOREIGN PATENTS OR APPUCATlONS formance built-in" antenna is achieved. 974.259 11/1960 Germany 325/365 317.566 1929 Great Britain 325/365 4 ClaImS, 2 Drawing Figures 300 w/M JAM/(ER TIA/M4 fE'AM/A/Al r TERM/N411 RIG/{7. 1 1 2 #[C'f/VIR mam/7 FM f/Z'Iid I IAN/IV TVA [K AMPl/F/[R L? a BUILT-IN FM DIPOLE ANTENNA BACKGROUND OF THE INVENTION Ever since radio receivers started to enjoy wide use, extensive efforts have been made to make the antenna circuits integral with the receivers. For standard broadcast band receivers, first the loop antenna became standard and then the ferrite loop made possible a small integrated package.

Similar success has not been achieved for higher frequency receivers such as FM broadcast receivers, which operate at a frequency two orders of magnitude above the standard broadcast band. Such receivers, and particularly those that are battery operated, typically employ collapsible dipole antenna elements. Where a-c line operation is used, a line-cord antenna is often employed but they are not always satisfactory. In large console receivers an actual dipole antenna is often tacked inside the cabinet. Since the length of a half wavelength antenna is about 1.5 meters in the middle of FM band (neglecting dielectric and end effects), it can be seen that a small table model or component type receiver cannot accommodate a full built-in dipole. The collapsible dipole, or rabbit ear antenna, is effective but is considered inconvenient and somewhat awkward to use. The collapsible monopole, which is used where the receiver has sufficient size to serve as an electrical ground plane or where an a-c line cord is present, has similar though lesser drawbacks. In general. small, component type receivers, particularly the battery operated versions, present a substantial problem in regard to built-in antenna operation.

SUMMARY OF THE INVENTION It is an object of the invention to employ the speaker cables in an FM stereo component receiver as a built-in dipole antenna.

It is a further object of the invention toprovide, in a stereo component type radio receiver, r-f coupling between the receivers speaker jacks and antenna terminals, while at the same time providing r-f isolation between such speaker jacks and the output terminals of the receivers audio frequency amplifier.

These and other objects are achieved in the following manner. The FM stereo component receiver is fitted with connecting leads that electrically locate the speaker jacks between Ill 6 and 3/16 wavelength from the amplifier chassis connection. Then coupling capacitors are connected between each speaker jack and an antenna terminal in a balanced input circuit. When the component speakers are in place, they are ordinarily located about one-half wavelength or more apart and the left and right speaker cables therefore act as the left and right arms, respectively, of a dipole antenna. Thus, through the application of this invention, a stereo component system becomes uniquely adapted for efficient built-in antenna operation.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a pictorial drawing showing an FM stereo component receiver employing the invention; and

FIG. 2 is a partial block-schematic diagram showing the essential features that are added to a conventional FM stereo receiver that permit it to utilize the left and right speaker cables as extended elements of an effective dipole antenna.

DESCRIPTION OF THE INVENTION FIG. 1 shows the basic elements of the invention. Receiver l is an FM stereo broadcast receiver component. It has a pair of antenna terminals 2 normally designed to present a balanced BOO-ohm impedance. In a typical component system left speaker 3 and right speaker 4 are separate and are remotely positioned to provide suitable stereo reproduction in the conventional manner. Cables 5 and 6 connect the speakers to the speaker terminals 7 and 8 located on the receiver. While the antenna terminals and the speaker terminals are shown on the top, these terminals are usually located on the rear panel of the receiver and are ordinarily in fairly close proximity to each other as will be described more particularly with reference to FIG. 2.

Typically cables 5 and 6 are supplied with the speaker system and are precut with permanent terminal fittings, these latter usually being molded plugs designed to mate with suitable jacks located on the speakers and on the receiver. These cables are often as much as 5 meters long'and are seldom less than one meter long. In typical operation, the user is instructed to locate the speakers for good aural effect and to coil the excess cable on the back of the speaker or into a pocket located therein. Thus a typical system will usually have the speakers two meters or more apart. While one speaker may be located at a greater distance from the receiver than the other speaker, their combined separation is substantial and normally adequate for dipole antenna performance. I

The speaker cables 5 and 6 are coupled respectively to the antenna terminals by capacitors 10 and 9.'These capacitors will typically have a value of about picofarads. The reactance of such a capacitor is quite low at FM broadcast frequencies and quite high at audio frequencies. These capacitors will coupled r-f energy but will not appreciably couple audio frequency energy and will have no effect upon the speaker signals. Thus the speaker cables form a dipole antenna of more than one quarter wavelength each and provide excellent signal pickup performance. Since dipole action is present, no ground plane is needed-Consequently, theantenna works well on verysmall component receivers.

While capacitors 9 and 10 are, for purposes of this illustration, shown as being located externally to the receiver, they preferably are located interior by and, if desired, can be permanently connected. If the permanent connection is used, an external antenna can be connected to terminals 2 in the usual manner. The antenna of the invention will have little effect on strong signals thus injected. More desirably, where an external antenna is available, and, in particular, whereweak signal reception is desired, capacitors 9 and 10 could be installed with plug-in connector fittings (not shown) at terminals 2.

FIG. 2 is a more detailed showing of the elements of receiver 1. The dashed outline represents the outline of the receiver cabinet. Speaker terminals are shown diagramat ically as a pair of conventional jacks, each with signal and common connections to the output terminals of tuner-amplifier 12. While the tuner-amplifier section 12 is shown as'a single unit, it can be composed of several individual circuit cards or elements connected to provide that desired function. A conventional balun I3, connectedbetween the antenna terminals and the tuner-amplifier 12, provides the proper balanced impedance to the antenna. Capacitors 9 and 10 are shown,

with interior mounting, connected between the speaker signal leads and the antenna terminals. Ordinarily the antenna terminals and the speaker jacks will be physically quite close together in the typical small modern component designs. The capacitor lead length and dress will not be critical. However, it is desirable that the length of wire from the speaker jacks or terminals to the component amplifier terminals be of a specified length 1. Length 1 is preferred to be about one eight wavelength. As a practical matter, I can be varied from about one sixteenth to three sixteenth wave length, or about 19 cm to 56 cm for the FM band. The action of the section labeled 1 is important because the tuneramplifier 12 will act as a ground plane to some degree, particularly when a-c line connected as shown. The tuner amplifier output should therefore be isolated from the antenna circuit. By making'about one eight wavelength, the speaker jacks will see a high impedance at r.f. looking back into the tuner-amplifier 12 output.

An improved built-in dipole antenna has been described and its use in connection with an FM stereo component receiver shown in detail. Clearly other embodiments within the scope of the invention are possible. Accordingly, it is intended that the scope of the invention be limited only by the following claims.

We claim:

1. A stereophonic radio receiver system comprising: a cabinet housing astereo broadcast receiver having balanced antenna input terminals and left and right speaker terminals, said receiver being capable of receiving stereo broadcast signals and producing related audio frequency output signals, a pair of loudspeakers, said loud speakers each mounted separately from said cabinet and adapted for location to provide stereo sound when provided with said signals from said receiver, means for connecting each of said pair of loudspeakers to respective ones of said speaker terminals, a first coupling capacitor located at said cabinet for connecting one of said antenna input terminals to one of said speaker terminals, a second coupling capacitor located at said cabinet for connecting the other of said antenna input terminals to the other of said speaker terminals, and means connected between said speaker terminals and said receiver for developing substantial impedance at r.f. while developing substantial no impedance at audio frequency.

2. The system of claim 1, wherein said first and second coupling capacitors have a value that will provide a very high reactance at audio frequencies and a low reactance at r-f broadcast frequencies.

3. The system of claim 2 wherein said receiver is incentimeters.

centimeters to about 56' 

1. A stereophonic radio receiver system comprising: a cabinet housing a stereo broadcast receiver having balanced antenna input terminals and ''''left'''' and ''''right'''' speaker terminals, said receiver being capable of receiving stereo broadcast signals and producing related audio frequency output signals, a pair of loudspeakers, said loud speakers each mounted separately from said cabinet and adapted for location to provide stereo sound when provided with said signals from said receiver, means for connecting each of said pair of loudspeakers to respective ones of said speaker terminals, a first coupling capacitor located at said cabinet for connecting one of said antenna input terminals to one of said speaker terminals, a second coupling capacitor located at said cabinet for connecting the other of said antenna input terminals to the other of said speaker terminals, and means connected between said speaker terminals and said receiver for developing substantial impedance at r.f. while developing substantial no impedance at audio frequency.
 2. The system of claim 1, wherein said first and second coupling capacitors have a value that will provide a very high reactance at audio frequencies and a low reactance at r-f broadcast frequencies.
 3. The system of claim 2 wherein said receiver is intended for FM broadcast use and the combined length of said connecting means is selected to be at least about 1.5 meters.
 4. The system of claim 3, wherein said speaker terminals are connected to said receiver by means of leads having a length of about 19 centimeters to about 56 centimeters. 